The present invention relates to a scanning electron microscope capable of observing a minute measuring object present on the surface of a specimen.
Microscopes using a probe of charged particles, including a scanning electron microscope, have been used for observation of a fine structure of a specimen in the field of research and development. In the scanning electron microscope, an SEM (scanning electron microscope) image of an observing object desired for observation is displayed on the screen. This technology is applied to observation of a fine structure of a semiconductor device and is indispensable for production process. In recent years, minuteness of the semiconductor device has been advancing and at present, a semiconductor device having a pattern width of 100 nm or less has been fabricated. In the semiconductor device as above, even in the presence of a foreign matter or defect of about several of tens of nanometers on a wafer on which a semiconductor pattern is formed, a fault will sometimes occur. With the object of examining in detail foreign matters or defects responsible for faults, observation and image-pickup of the foreign matters or defects with the scanning electron microscope or the like is often conducted. Available as this type of apparatus is a defect inspection apparatus or an inspection apparatus called a review SEM.
Supposedly, such an inspection apparatus as above is placed in the production line of semiconductor device and brought into unattended operation. Further, to assist in speeding up inspection, individual processing times need to be as short as possible and an image must be picked up at high speeds. When picking up a clear image removed of focus blur, it is general to make focus adjustments automatically by using the function of, for example, autofocus and then pick up images. In the case of the scanning electron microscope, a method is often employed for autofocus, according to which while changing the focal position, an electron probe is irradiated synchronously with scanning of display and a focal position is calculated on the basis of obtained signals such as secondary electron signals. In an exemplary method of this type, the focal position is changed to obtain images at individual focal positions, the thus obtained images are processed through a high-pass filter and intensities of the filtered images are plotted on a graph to determine an in-focus position corresponding to a maximally intensive point. In this procedure, data is acquired while moving the focal position and therefore this process is time-consuming. For efficient inspection, it is necessary that the time consumed for autofocus be as short as possible, the number of autofocus operations be reduced and many defects be picked up within a short period of time. For reduction of the number of autofocus operations, a method has been known in which the wafer surface height is detected with an optical type height detector and focus of an SEM is settled on the basis of a detected surface height (for example, see JP-A-2002-310962 (U.S. Pat. No. 6,657,221). In the electron microscope, however, an electron beam is used as a probe and consequently, the wafer surface will sometimes be charged and under the influence of charged electricity, the trajectory of electrons will sometimes be deflected, so that electrons cannot be focused on a scheduled point and a defocus results, giving rise to a blurred image. The charged electricity cannot be measured with the height detector and causes a serious problem during image pickup. A technique aiming at correction of the defocus has been known in which before measurement, a value of charged potential on a wafer is measured with an electrostatic meter and a defocus presumable from a quantity of charged electricity is corrected or in-focus positions are measured in advance at several locations on the wafer, thus preparing a curved surface for interpolation and then the current value of an objective lens is controlled in accordance with the curved surface to thereby reduce the number of autofocus operations (see JP-A-05-003013, for example). But, in these conventional techniques, as the electric charge condition changes with time, correction cannot sometimes be made successfully by merely using initially acquired data.